The present invention relates to improvements in electronic cameras, or picture signal generating apparatus, of the type having an image tube including a photocathode for receiving incident image-conveying radiation, and a charge storage target such as the silicon target of a silicon target vidicon for storing an image-representing charge pattern on the target. More particularly, the invention relates to such apparatus having improved means for compensating for excessive variations in incident radiation intensity, by providing monitoring and control of the intensity of the image-conveying radiation arriving at the photocathode, coordinated with monitoring and control of the degree of acceleration of photoelectrons between the photocathode and target.
The invention contemplates that the image-conveying radiation incident at the photocathode may be of various kinds, such as visible light, infra-red radiation, X-rays, ultraviolet, and the like, and the efficacy of the invention is not limited to any particular wavelength or class of such image-conveying radiation, to all of which radiation, for simplicity, the term "light" will hereinafter be generically applied. The invention has particular utility in applications where the light level of the scene being viewed by the camera is subject to wide variations in intensity, such as from bright to dark night.
One of the problems which the present invention provides a particularly effective solution for is temporary or permanent reduction in the sensitivity of the image tube when the scene light level suddenly increases. This problem can take various forms. For example, excessive light impinging on the photocathode can cause excessive photo-emission which in turn can cause contamination of the photocathode by ionized residual gas within the intensifier section. The excessive photoelectron flow may also cause permanent degradation of the elements of the image tube. Another problem at high light levels is that corrective reduction of photoelectron accelerating voltage can cause nonuniformity in sensitivity over the image area of the photocathode which in turn reduces picture quality.
To attempt to minimize the possiblity of such undesired results, various techniques have heretofore been used, such as an arrangement to automatically introduce an incident light intensity-limiting filter, such as a neutral-density filter, into shielding or attenuating relation to the photocathode when the incident light intensity exceeds a predetermined level. Other prior art forms of such protective attempts have involved automatic control of an incident light transmitting aperture or iris in the incident light path to the photocathode. However, such prior art protective arrangements have the disadvantage of requiring light intensity sensor means ahead of, i.e. external to, the image tube, an arrangement which is not only relatively expensive but even more importantly interferes with and degrades the operating characteristics of the image tube. Also, such external sensor arrangements do not have the capability which the present invention has of optimizing the level of the incident light relative to the requirements of the photocathode, for a given scene viewed by the camera.
Accordingly, a principal object of the present invention is to provide improved means, in picture signal generating apparatus of the character hereinabove described, for protecting against excessive incident light levels without diminishing output picture quality throughout the designed light level operating range of the image tube.
another object is to provide an improved light intensity variation compensating means for image tubes of the character described, including coordinated control of both photocathode incident light and intensifier photoelectron acceleration.
Another object is to provide image tube control means of the character described which does not require light sensors other than the image tube itself, and which enables the full operating range of the image tube to be utilized while preserving picture quality and avoiding damage to the image tube throughout such range.
These and other objects of the present invention will be apparent from the following description together with the accompanying drawing.